1. Field of the Invention
The present invention relates to a novel chromene compound which changes into a colored state upon irradiation with light containing ultraviolet rays such as of sunlight or light of a mercury lamp, the change being reversible, and exhibits excellent durability.
2. Prior Art
Photochromism is a phenomenon which is drawing attention in these several years, and stands for a reversible action of a compound; i.e., a compound quickly changes its color when it is irradiated with light containing ultraviolet rays such as of sunlight or light of a mercury lamp, and resumes its initial color when it is no longer irradiated with light and is placed in a dark place. The compound having such a property is called photochromic compound. Though a variety of compounds have heretofore been synthesized, no common feature is particularly recognized among their structures.
U.S. Pat. No. 3,567,605 discloses a chromene compound represented by the following formula, ##STR2##
This chromene compound exhibits photochromic property near room temperature (20 to 30.degree. C.) but develops a color of only a low density upon the irradiation with ultraviolet rays, and is not practicable.
Furthermore, WO9422850 discloses a chromene compound represented by the following formula, ##STR3##
This compound develops a color of a higher density than the compound disclosed in the above-mentioned U.S. Pat. No. 3,567,605. However, when dispersed in a solvent or a matrix, the compound partly develops a color, i.e., initially colored to a large extent and is not practicable.